docs: def check_cv(cv, X=None, y=None, classifier=False):
      """Input checker utility for building a CV in a user friendly way.
  
      Parameters
      ----------
      cv : int, a cv generator instance, or None
          The input specifying which cv generator to use. It can be an
          integer, in which case it is the number of folds in a KFold,
          None, in which case 3 fold is used, or another object, that
          will then be used as a cv generator.
  
      X : array-like
          The data the cross-val object will be applied on.
  
      y : array-like
          The target variable for a supervised learning problem.
  
      classifier : boolean optional
          Whether the task is a classification task, in which case
          stratified KFold will be used.
  
      Returns
      -------
      checked_cv: a cross-validation generator instance.
          The return value is guaranteed to be a cv generator instance, whatever
          the input type.
      """
      return _check_cv(cv, X=X, y=y, classifier=classifier, warn_mask=True)